Java 线程中断标志被清除

Question

I was experimenting with interrupts in java and observed that if i interrupt a thread and do not reset it's interrupt status when the thread state changed from RUNNABLE to TERMINATED interrupted variable gets reset (interrupted becomes false)

Main.java

public class Main {

public static void main(String[] args) {

    try {

        t1 t1 = new t1();
        t1.start();

        Thread.sleep(10);
        t1.interrupt();
        System.out.println("t1.isInterrupted: " + t1.isInterrupted() + " t1.state: " + t1.getState());
        Thread.sleep(20);
        System.out.println("t1.isInterrupted: " + t1.isInterrupted() + " t1.state: " + t1.getState());

    } catch (Exception exc) {
        System.out.println("Exception: " + exc);
    }

}

}

t1.java

public class t1 extends Thread {

@Override
public void run() {

    while (!Thread.currentThread().isInterrupted()) {
        System.out.println("Processing");
    }

    System.out.println("Execution completed - interrupt status: " + Thread.currentThread().isInterrupted());
}

}

Output of Main.java (Centos 7 64 bit, Java 1.8.0_66-b17)

• Processing
  Processing
  Processing
  t1.isInterrupted: true t1.state: RUNNABLE
  Execution completed - interrupt status: true
  t1.isInterrupted: false t1.state: TERMINATED

My question is who or what sets this flag back to false and why ? Thread class has a private exit method that sets some internal variables to null but i couldnt find anything that resets interrupted flag. If anyone can shed some light on the issue i will be grateful ...

PS: here is a link to GIST

Answer 1

My question is who or what sets this flag back to false and why ?

This is the known behavior when the thread terminates. Because it is no longer running, the interrupt state is false. The interrupt state is true right before the thread terminates but is set to false once it has completed.

To quote the Thread.isInterrupted() javadocs :

A thread interruption ignored because a thread was not alive at the time of the interrupt will be reflected by this method returning false.